(a) Field of the Invention
The present invention relates to a method for preparing a super absorbent polymer. More specifically, the present invention relates to a method for preparing a super absorbent polymer having improved permeability by adding a polymer particle during surface crosslinking.
(b) Description of the Related Art
Super absorbent polymer (SAP) is a synthetic polymer material having a function capable of absorbing moisture from about 500 to about 1,000 times of its own weight. Each manufacturer has denominated it as different names such as SAM (Super Absorbency Material), AGM (Absorbent Gel Material) or the like. Such super absorbent polymers started to be practically applied in sanitary products, and now they have been widely used not only for hygiene products such as disposable diapers for children, etc., but also for water retaining soil products for gardening, water stop materials for the civil engineering and construction, sheets for raising seedling, fresh-keeping agents for food distribution fields, materials for poultice or the like.
As a method for preparing a super absorbent polymer, an inverse suspension polymerization method, an aqueous solution polymerization method or the like are known. For example, the inverse suspension polymerization is disclosed in Japanese Patent Publication Nos. Sho56-161408, Sho57-158209, Sho57-198714, and so on. Furthermore, the aqueous solution polymerization method, such as a thermal polymerization method in which a polymer gel is polymerized while being broken and cooled in a kneader equipped with a plurality of spindles, and a photo polymerization method in which an aqueous solution with a high concentration is irradiated with UV rays or the like onto a belt to carry out the polymerization and drying at the same time are known.
The hydrogel polymer obtained through said polymerization reaction is generally marketed as a powdered product after it is dried and pulverized.
In a product using the super absorbent polymer, the permeability is an index for measuring a fluidity of a liquid to be absorbed. The permeability may vary depending on characteristics such as a particle size distribution of a crosslinked resin, a shape of the particle and a connectivity of the openings between the particles, a surface modification of a swollen gel and the like. The fluidity of the liquid passing through the swollen particles may change depending on the permeability of a super absorbent polymer composition. When the permeability is low, it does not enable the liquid to easily flow through the super absorbent polymer composition.
One method for increasing a permeability of the super absorbent resin may include, for example, a method for polymerizing a resin and then carrying out a surface crosslinking reaction. In this case, a method of adding silica, clay or the like together with a surface crosslinking agent has been utilized. For example, U.S. Pat. Nos. 5,140,076 and 4,734,478 disclose the addition of silica during surface crosslinking of a dried super absorbent polymer powder.
However, there were problems in that the permeability is improved due to the addition of silica, clay or the like, but it shows that a water holding capacity and an absorption power under pressure are decreasing in proportion thereto, and it is easy to be separated from the super absorbent polymer by external physical impact during movement.